Method for controlling the oil recirculation in an oil-injected screw-type compressor and compressor using this method

ABSTRACT

The invention relates to a method for controlling the oil recirculation in an oil-injected screw-type compressor which, between the oil separator ( 10 ) and the compressor element ( 1 ), comprises an oil recirculation conduit ( 17 ) in which an oil cooler ( 18 ) is installed which is bridged-over by a passage or bypass ( 30 ), said control taking place by means of a thermostatic valve ( 24 ) having a valve element ( 26 ) which can be moved by means of a temperature-sensitive element ( 34 ). During the transition of the screw compressor from the unloaded to the loaded condition, the effect of the temperature-sensitive element ( 34 ) temporarily is switched off at least partially, such that the valve element ( 26 ) takes a position in which, regardless of the temperature of the oil, at least the bypass ( 30 ) is open and thus the recirculation of oil from the oil separator ( 10 ) to the compressor element ( 1 ) takes place through this bypass ( 30 ).

This invention relates to a method for controlling the oil recirculationin an oil-injected screw-type compressor comprising a compressorelement, connected thereto an inlet conduit and a pressure conduit, anoil separator in said pressure conduit, an oil recirculation conduitbetween said oil separator and the compressor element, in whichrecirculation conduit an oil cooler is installed, and a bypassbridging-over the oil cooler in the recirculation conduit, whichcontrolling is performed by means of a thermostatic valve having a valveelement which can be moved by means of a temperature-sensitive element,whereby the temperature-sensitive element measures the temperature ofthe recirculating oil and the valve element, if this temperature isbelow a certain value, opens the bypass, such that the separated oilfrom the oil separator can flow directly towards the compressor elementwithout having to flow over the oil cooler and, if the temperature ofthe oil is above a certain value, which is higher than or equal to theaforementioned value, the valve element closes off the bypass.

According to the known methods, the valve element of the thermostaticvalve is in that position in which it opens the bypass, when the oil iscold, and this when the compressor is without load as well as when thecompressor changes from the unloaded to the loaded status.

When the oil is warmer than a well-defined temperature, then the valveelement is in that position in which it closes the bypass, as aconsequence of which the oil from the oil separator is forced to flowover the oil cooler before being injected back into the compressorelement.

When the compressor is running without load and thus no air is suctionedinto the compressor element, the pressure in the oil separator, whichlatter also serves as a pressure vessel, is kept as low as possible inorder to limit the unloaded power consumption.

When transiting into the loaded working condition, and thus when openingthe inlet valve, the screw-type compressor element maximally suctionsair which then is compressed. Due to the low pressure in the oilseparator, the oil pressure at the beginning of the transition also islow.

When the oil temperature is high, the bypass thus is closed, such thatthe oil flows over the oil cooler, which moreover causes a pressuredrop, such that the oil injection pressure temporarily is particularlylow.

As a consequence, with these known methods high temperature peaks may becreated at the outlet of the compressor element.

The pressure in the oil separator during the unloaded operation of thecompressor element and, thus, the consumed input, can not be chosenoptimally low in order to prevent the occurrence of said temperaturepeaks.

The invention aims at a method for controlling the recirculation of theoil, whereby the pressure in the oil separator, when the compressorelement is working without load, can be kept lower, without the risk oftemperature peaks at the outlet of this compressor element during thetransition from unloaded to loaded operation.

According to the invention, to this aim, during the transition from theunloaded to the loaded condition of the screw-type compressor, theinfluence of the temperature-sensitive element temporarily is switchedoff at least partially, such that the valve element temporarily takes aposition in which, regardless of the temperature of the oil, at leastthe bypass is open and thus the recirculation of oil from the oilseparator towards the compressor element temporarily takes place atleast by means of this bypass.

Thus, the additional pressure drop in the oil cooler temporarily isswitched off, such that, notwithstanding the low pressure of the oil,there still is a sufficient injection pressure in order to avoidtemperature peaks at the outlet of the compressor element.

This switching-off of the influence of the temperature-sensitive elementis solely of a short duration, in consideration of the fact that, underload, the pressure in the oil separator rapidly increases.

When transiting from unloaded to loaded, the valve element preferablytakes a position whereby the bypass as well as the recirculation conduitare open, such that the oil temporarily can flow back to the compressorelement through the bypass as well as through the oil cooler, regardlessof the temperature of the oil.

The temporarily, at least partially, switching-off the effect of thetemperature-sensitive element can take place by realizing a part of thewall of the thermostatic valve, against which the temperature-sensitiveelement normally is situated, as a piston of a pneumaticallycontrollable piston mechanism, whereby the temperature-sensitive elementcan expand without moving the valve element, for example, by pushingaway this piston, and whereby, for example, the pressure in the oilseparator and the control pressure for operating a controlled inletvalve in the inlet conduit are used as control pressures.

The invention also relates to an oil-injected screw-type compressorwhich is suitable for being controlled according to the method describedin the aforegoing.

Thus, the invention also relates to an oil-injected screw-typecompressor comprising a screw-type compressor element, connected theretoan inlet conduit and a pressure conduit, an oil separator in saidpressure conduit, an oil recirculation conduit between said oilseparator and the compressor element, in which recirculation conduit anoil cooler is arranged, and a bypass bridging-over the oil cooler in therecirculation conduit and which can-be closed off by means of a valveelement of a thermostatic valve with a valve element that can be movedby a temperature-sensitive element situated in the recirculationconduit, and with as a characteristic that the screw-type compressorcomprises a control system which, when transiting from the unloaded tothe loaded condition, temporarily switches off the effect of thetemperature-sensitive element, onto the valve element of thethermostatic valve at least partially, such that during this transition,the valve element is in a position whereby at least the bypass is open,regardless of the temperature of the oil.

The bypass can be limited to a passage between a part of therecirculation conduit situated between the oil separator and the oilcooler, and a part of the recirculation conduit situated between the oilcooler and the compressor element.

In a particular form of embodiment of the invention, the valve elementof the thermostatic valve is situated in the bypass as well as in therecirculation conduit upstream from the bypass, such that, in oneposition, it simultaneously opens the bypass and closes off the part ofthe recirculation conduit situated between the outlet of the oil coolerand the bypass, in another position simultaneously closes off the bypassand further opens the aforementioned part of the recirculation conduit,and preferably in the first-mentioned position and/or in an intermediateposition opens the bypass as well as opens the aforementioned part ofthe recirculation conduit.

The valve element takes up the first-mentioned position, amongst others,when, during the transition from the unloaded to the loaded condition,the working of the thermostatic valve is switched off at leastpartially.

The aforementioned control system may comprise a piston mechanism, thepiston of which, in a well-defined position, forms a stop for thetemperature-sensitive element. When this piston is freely movable, thenthe temperature-sensitive element of the thermostatic valve can freelychange its length, and the effect of this thermostatic valve thus isswitched off at least partially.

With the intention of better showing the characteristics of theinvention, hereafter, as an example without any limitative character, apreferred form of embodiment of a method for controlling the oilrecirculation in an oil-injected screw-type compressor and screw-typecompressor controlled in this manner, according to the invention, isdescribed, with reference to the accompanying drawings, wherein:

FIG. 1 schematically represents a screw-type compressor according to theinvention, during cold starting;

FIG. 2, in cross-section and at a larger scale, represents a practicalembodiment of the part indicated by F2 in FIG. 1;

FIG. 3 represents the screw-type compressor of FIG. 1, however, duringthe normal regime operation, either loaded or unloaded, when the oil iswarm;

FIG. 4, in cross-section and at a larger scale, represents a practicalembodiment analogous to that from FIG. 2, of the part indicated by F4 inFIG. 3;

FIG. 5 represents the screw-type compressor during the transition fromunloaded to loaded operation, when the oil still is warm;

FIG. 6, in cross-section and at a larger scale, represents a practicalembodiment analogous to that of FIGS. 2 and 4, of the part indicated byF6 in FIG. 5;

FIG. 7 represents a cross-section analogous to that from FIGS. 2, 4 and6, however, relating to another status of the screw-type compressor.

The screw-type compressor represented in the figures comprises acompressor element 1 comprising a housing 2 surrounding a rotor chamber3 in which two mutually cooperating screw-shaped rotors 4 are installed.The compressor element 1 is driven by a motor, not represented in thefigures.

At the inlet side, an inlet conduit 5 gives out in the rotor chamber 3,in which conduit 5 air filters 6 and a controlled inlet valve 7 areprovided, whereas at the outlet side, a pressure conduit 8, by means ofan outlet valve 9 which, for example, is a return valve, connects to therotor chamber 3.

In the pressure conduit 8, successively an oil separator 10, an aircooler 11, and a water separator 12 are arranged.

In the oil separator 10, there is a vessel 13 which is provided with anoutlet 14 at the top. Opposite to outlet 14, a filter 15 is installed inthe vessel 13, and a minimum pressure valve 16 is installed in theoutlet 14.

The major part of the oil is collected in the lower part of the vessel13, and the underside of vessel 13 is connected to an injection point ofthe compressor element 1 by means of a recirculation conduit 17.

In this recirculation conduit 17 for the oil, successively an oil cooler18, an oil filter 19 and a controlled oil valve 20 are provided.

For control, the oil valve 20, by means of a control conduit 21, is inconnection with the outlet of the compressor element 1.

By means of a conduit 22, the interior of the filter 15 is in connectionwith the interior of the rotor chamber 3 for recirculating the oilcollected at the bottom of the filter 15.

The oil cooler 18 and the air cooler 11 are cooled by a common fan andhave radiators which are united to one single block.

The oil filter 19 is provided on the housing 23 of a thermostatic valve24. This valve 24 comprises a space 25 in which a valve element 26 issituated and a space 28 separated therefrom by a partition 27.

The space 25 is in connection with the inlet of an oil filter 19 placedon the housing 23 and thus is situated in the recirculation conduit 17.This space 25 forms the connection between said oil filter 19 and thepart 17B of the recirculation conduit 17 situated between the outlet ofthe oil cooler 18 and the housing 23. The connection of the part 17B tothe space 25 forms a passage 29 which can be closed off by the valveelement 26.

A bypass having the form of a passage 30 from the part 17C of therecirculation conduit 17, situated between the oil separator 10 and theinlet of the oil cooler 18, to the space 25 gives out into the space 25.This passage 30, too, can be closed off by the valve element 26.

The bypass for the oil bridges-over the oil cooler 18, and through thisbypass or passage 30, oil can flow directly from the oil separator 10 tothe oil filter 19 and further to the compressor element 1 withoutpassing through oil cooler 18.

When the valve element 26 closes off the passage 30 and thus the bypass,it opens the passage 29, and reverse, when the valve element 26 opensthe passage 30, it closes off the passage 29. In an intermediateposition, the valve element 26 leaves open both passages 29 and 30.

The space 28 is in connection with, on one hand, the outlet of thefilter element of the oil filter 19 and, on the other hand, the part 17Aof the recirculation conduit 17 situated between the oil filter 19 andthe oil valve 20.

As is represented more detailed in FIGS. 2, 4, 6 and 7, the thermostaticvalve 24 can be composed as follows:

The valve element 26 is a bush which is axially movable in a bore 25Awhich forms part of the space 25 and into which ring-shaped chambers 31and 32 give out, which respectively form part of the passages 29 and 30to which the conduit parts 17B and 17C connect.

The valve element 26 is provided with a slot 33 extending over a part ofthe circumference parallel to the chambers 31 and 32 and being smallerthan the width of the chambers 31 and 32 in axial direction.

A temperature-sensitive element 34 is axially installed in the valveelement 26, said element 34 having a base 35 and a finger 36 moving outof it when the temperature increases.

Normally, the finger 36 cooperates with a stop which is movable andwhich, in the represented example, is formed by a piston 37 which issituated in the prolongation of the bore 25A.

This piston 37 forms part of a control system 38 which shall bedescribed in the following.

The base 35 is attached to the valve element 26 by the intermediary of adisk ring 39.

A spring 40, which is provided between said disk ring 39 and a collar25B of the wall of the bore 25A, pushes the valve element 26 into thedirection of the housing 41 of the control system 38.

Said piston 37 consists of a plunger 37A fitting into an opening 42 inthe housing 41, and a head 37B with larger diameter situated in achamber 43 in the housing 41.

At the plunger side of the head 37B, the chamber 43, by means of a duct44, is in connection with the atmosphere.

At the other side of the head 37B, the chamber 43, by means of a duct45, connects to a conduit 46 ending up in the vessel 13.

This duct 45 can be put into connection with the atmosphere by means ofan auxiliary control, formed by a relief valve 47. Said relief valve 47comprises a valve body 48 having a hollow part provided with radialopenings 49 in its wall, which, for one position of this valve body 48,connects the duct 45, through the interior of this last-mentioned valvebody 48, to the atmosphere.

A part of the duct 45 forms a ring-shaped duct 45A around the bore 50for this valve body 48, and for said position of the valve body 48, theopenings 49 give out onto this ring-shaped duct 45A.

Whereas the interior of the valve body 48 at one extremity, by means ofa chamber 51 and a duct 52 in the housing 41, is in connection with theatmosphere, the hollow valve body 48 is closed off at the otherextremity and has a piston-forming part 48A which is movable in acylinder-forming chamber 53.

The most outwardly situated extremity of this chamber 53 connects, bymeans of a duct 54, to a control conduit 55 which is in connection withthe control conduit 55A for supplying the control pressure P1 to theinlet valve 7. By means of a not represented duct, the other extremityof the chamber 53 is in connection with the atmosphere.

In the chamber 51, two springs 56 and 57 are arranged which counteractthe movement of the valve body 48 under the influence of this controlpressure P1, to wit a relatively weak spring 56 between this valve body48 and the end of a tubular element 58, and a stronger spring 47 whichis provided around the tubular element 58 between a collar of thetubular element 58 and the extremity of the chamber 51.

The control of the recirculation of oil from the vessel 13 to thecompressor element 1 takes place as follows:

When the screw-type compressor is at rest, the inlet valve 7 is closedand there is no control pressure P1. The part 48A of the valve body 48is situated against the extremity of the chamber 53, and the openings 49are closed off by the housing 41.

The pressure P2 in the oil separator 10 is situated minimum 0,6 barabove atmospheric pressure, such that the piston 37 is pushed intowithdrawn position, whereby its end surface forming a stop for thefinger 36 is situated in the plane of the end of the bore 25A, asrepresented in FIGS. 2 and 4.

When the oil flowing from the oil separator 10 back to the compressorelement 1 has a temperature which is lower than a well-defined value,as, for example, with a first start before the compressor is put underload, then the finger 36 is slid maximally into the base 35, this isuntil the widened extremity of the finger 36 is situated against thebase 35, as represented in FIG. 2. Hereby, the valve element 26 is inthe position in which the passage 29 is closed off and the passage 30 isopen.

The oil flows from the oil separator 10, through the passage 30 and thuswithout being cooled in the oil cooler 18, to the compressor element 1,as represented by arrows in FIGS. 1 and 2.

When the temperature of the oil increases, then thetemperature-sensitive element 34 becomes longer and the finger 36 ispushed out of the base 35, which means that, considered that the piston37 does not change its position by the pressure P1, the base 35 is movedaway from the piston 37. By means of the disk ring 39, the base 36 takesalong the valve element 26, against the effect of spring 40. At awell-defined moment, this valve element 26 will leave open both passages29 and 30.

Once the oil has reached its normal operation temperature, then thefinger 36 is slid out maximally, and the condition represented in FIGS.3 and 4 is obtained. The valve element 26 closes off the passage 30entirely, whereas the passage 29 is maximally open. All of the oil flowsback through oil cooler 18, as represented by arrows in FIGS. 3 and 4.

At the moment that the control of the compressor gives a signal for thetransition from unloaded to loaded condition, in other words, whencompressed air has to be delivered, the pressure P2 prevailing in theoil separator 10, by means of control conduit 55A, is immediately usedas control pressure P1 of the inlet valve 7. In the chamber 53, thus acontrol pressure P1 prevails which is equal to the pressure P2 in theoil separator 10. This control pressure P1 is sufficiently high in orderto move the valve body 48 against the force of the weakest spring 56,however, is insufficient in order to equally compress the strongerspring 57. Thereby, the valve body 48 takes a position as represented inFIG. 6, whereby the openings 49 give out onto the duct 45.

Consequently, the chamber-43 temporarily is in connection with theatmosphere and the piston 37 in fact is free, and thetemperature-sensitive element 34 can push the piston 37 away. Under theinfluence of the spring 40, the valve element 26, as represented in FIG.6, will be pushed against the end of bore 25A, whereby the passage 29 aswell as the passage 30 are open and the oil thus can flow through theoil cooler 18 as well as through the bypass or passage 30. At thatmoment, the inlet valve 7 still is closed.

From FIG. 6, it is obvious that the valve element 26 takes said positionregardless whether the oil is cold or warm. When thetemperature-sensitive element 34, as a result of the warm oil, has amaximum length, it simply pushes the piston 37 further into the chamber43, as represented in FIG. 6.

The pressure P2 in the oil separator 10 increases continuously until itis high enough to open the inlet valve 7. At this stage, the risk is thelargest that temperature peaks occur in the compressor element 1 becauseof insufficient oil lubrication as a result of too low an oil pressureP2. Due to the fact that the oil, as represented by arrows in FIG. 6,can flow through the passage 30 and the chamber 25 directly to thecompressor element 1, the pressure drop in the oil cooler 18 is avoided,as a result of which a higher pressure is obtained at the inlet of theoil valve 20 and whereby thus a better oil lubrication is obtainedduring said transition stage from unloaded to loaded operation of thescrew-type compressor.

After opening the inlet valve 7, the pressure P2 in the oil separator 10and thus also the control pressure P2 increases more rapidly. When thecontrol pressure P1 is sufficiently high, the valve body 48, against theeffect of the stronger spring 57, is moved further up into the positionrepresented in FIG. 7. The passages 49 then are closed off by thehousing 41.

The part of the chamber 43 onto which the duct 45 gives out, then nolonger is in connection with the atmosphere, but is at the pressure P2.

Thereby, the piston 37 is pushed into its position represented in FIG.7, whereby the plunger 37A fills the opening 42 and forms a stop in theplane of the end of the bore 25A.

The pressure of the oil in the chamber 25, however, also isapproximately equal to P2, however, this pressure is exerted onto asmaller surface, to wit that of the plunger 37A, than the surface of thehead 37B.

As the oil is at operation temperature, the finger 36 of thetemperature-sensitive element 34 is maximally pushed out, as a result ofwhich the valve element 26, against the effect of the spring 40, isbrought into the position represented in FIG. 7.

This valve element 26 then closes off the passage 30, whereas thepassage 29 is open. The oil flows as is represented by arrows in FIGS. 3and 7, this is through the part 17C of the conduit 17 to the oil cooler18 and from there through the part 17B and through the passage 29 to thefilter 29.

When the load of the compressor, which by now is warmed up, stops, thenfirst the inlet valve 7 is closed and the control pressure P1 dropsbelow said minimum value, as a result of which the valve body 48, by thesprings 56 and 57, is pushed back to the position represented in FIGS. 3and 4.

The pressure P2 in the oil separator and thus also the pressure of theoil effecting on the piston 37, drops to a minimum value, whichnevertheless still is sufficient for keeping the piston 37 pushed in,such that the condition from FIG. 4 is obtained and the warm oil, asrepresented in FIG. 3, must flow through oil cooler 18.

When the compressor again changes from the unloaded to the loadedcondition, the process described heretofore in connection with suchtransition is repeated.

Thus, this means that with each transition from the unloaded to theloaded condition of the compressor, when the oil pressure is low, thepassage 30, as represented in FIG. 6, is temporarily opened and thus theoil substantially can flow through the bypass formed by this passage 30directly from the oil separator 10 to the filter 19 and from there tothe oil valve 20, whereby an additional pressure drop over the oilcooler 18 is avoided.

In that during the transition, as also represented in FIG. 6, passage 29is open, too, the oil also will partially, however, to a lesser extent,flow through the oil cooler 18, as a result of which, at the end of saidtransition phase, when the passage 30 suddenly is closed off and themaximum oil flow rate must flow through the oil cooler 18, the oil flowrate through this oil cooler 18 will increase less sudden and thetransition thus will take place at a steadier pace.

As with each transition from an unloaded to a loaded condition, eachtime the oil cooler 18 is bypassed, the pressure drop in the oil issmaller, as a result of which the oil is injected into the compressorelement 1 at a higher pressure and consequently a better lubrication isobtained, such that the risk of temperature peaks at the outlet of thecompressor element 1 diminishes.

According to the same argumentation, it can be stated that duringunloaded operation, the oil pressure in the oil separator 10 may droplower than in a classical compressor without control system 38 accordingto the invention, without the risk of such damaging temperature peaks.

The invention is in no way limited to the form of embodiment describedin the aforegoing and represented in the accompanying drawings, however,such method for controlling the oil recirculation in an oil-injectedscrew-type compressor and such controlled screw-type compressor can berealized in various variants, without leaving the scope of theinvention, as determined by the accompanying claims.

1. Method for controlling the oil recirculation in an oil-injectedscrew-type compressor comprising a compressor element (1), connectedthereto an inlet conduit (5) and a pressure conduit (8), an oilseparator (10) in said pressure conduit (8), an oil recirculationconduit (17) between said oil separator (10) and the compressor element(1), in which recirculation conduit (17) an oil cooler (18) isinstalled, and a passage or bypass (30) bridging-over the oil cooler(18) in the recirculation conduit (17), which controlling is performedby means of a thermostatic valve (24) having a valve element (26) whichcan be moved by means of a temperature-sensitive element (34), wherebythe temperature-sensitive element (34) measures the temperature of therecirculating oil and the valve element (26), if this temperature isbelow a certain value, opens the bypass (30), such that the separatedoil from the oil separator (10) can flow directly towards the compressorelement (1) without having to flow over the oil cooler (18) and, if thetemperature of the oil is above a certain value, which is higher than orequal to the aforementioned value, the valve element (26) closes off thebypass (30), characterized in that during the transition of the screwcompressor from the unloaded to the loaded condition, the effect of thetemperature-sensitive element (34) temporarily is switched off at leastpartially, such that the valve element (26) temporarily takes a positionin which, regardless of the temperature of the oil, at least the bypass(30) is open and thus the recirculation of oil from the oil separator(10) to the compressor element (1) temporarily takes place at leastthrough this bypass (30).
 2. Method according to claim 1, characterizedin that during the transition from unloaded to loaded, the valve element(26) temporarily takes a position whereby the bypass (30) as well as therecirculation conduit (17) are open, such that the oil temporarily canflow through the bypass (30) as well as through the oil cooler (18) backto the compressor element (1), regardless of the temperature of the oil.3. Method according to claim 1, characterized in that the temporarilyswitching off of the effect of the temperature-sensitive element (34)takes place by realizing a part of the wall of the thermostatic valve(24), against which the temperature-sensitive element (34) normally issituated, as a piston (37) of a pneumatically controllable pistonmechanism.
 4. Method according to claim 1, characterized in that ascontrol pressures, the pressure (P2) in the oil separator (10) and thecontrol pressure (P1) for operating a controlled inlet drive (7) in theinlet conduit (5) are used.
 5. Method according to claim 1,characterized in that the pressure (P2) prevailing in the oil separator(10) is exerted onto a head (37B) of the piston (37), whereas the oilpressure itself is exerted onto an end of the piston (37) with a smallersurface, said end forming a plunger (37A), and the side along which thefirst-mentioned pressure (P2) is exerted, can be put into connectionwith the atmosphere by means of an outlet (52) which is controlled by avalve body (48) which in its turn is controlled by the control pressure(P1) of a controlled inlet valve (7).
 6. Oil-injected screw-typecompressor comprising a screw-type compressor element (1), connectedthereto an inlet conduit (5) and a pressure conduit (8), an oilseparator (10) in said pressure conduit (8), an oil recirculationconduit (17) between said oil separator (10) and the compressor element(1), in which recirculation conduit (17) an oil cooler (18) isinstalled, and a bypass (30) bridging-over the oil cooler (18) in therecirculation conduit (17) and which can be closed off by the valveelement (26) of a thermostatic valve (24) with a valve element (26)which can be moved by means of a temperature-sensitive element (34)situated in the oil recirculation conduit (17), characterized in thatthe screw-type compressor comprises a control system (38) which, duringthe transition from the unloaded to the loaded condition, temporarilyswitches off the effect of the temperature-sensitive element (34) ontothe valve element (26) of the thermostatic valve (24) at leastpartially, such that during this transition, the valve element (26) isin a position in which at least the bypass (30) is open, regardless ofthe temperature of the oil.
 7. Screw-type compressor according to claim6, characterized in that the bypass (30) is limited to a passage (30)between a part (17C) of the recirculation conduit (17) situated betweenthe oil separator (10) and the oil cooler (18), and a part (17B) of therecirculation conduit (17) situated between the oil cooler (18) and thecompressor element (1).
 8. Screw-type compressor according to claim 6,characterized in that the valve element (26) of the thermostatic valve(24) is situated in the bypass (30) as well as in the recirculationconduit (17) upstream from the bypass (30), such that, in one position,it simultaneously opens the bypass (30) and closes off the part (17B) ofthe recirculation conduit (17) situated between the outlet of the oilcooler (18) and the bypass (30), and in another position simultaneouslycloses off the bypass (30) and opens said part (17B) of therecirculation conduit (17).
 9. Screw-type compressor according to claim8, characterized in that the valve element (26) in the first-mentionedposition and/or in an intermediary position opens the bypass (30) aswell as said part (17B) of the recirculation conduit (17). 10.Screw-type compressor according to claim 7, characterized in that thethermostatic valve (24) comprises a housing (23) with a space (25)inside, in which space a valve element (26) is movable, and that thepassage (30) is an opening giving out onto this space (25). 11.Screw-type compressor according to claim 10, characterized in that itcomprises an oil filter (19) which is installed in the recirculationconduit (17), between the bypass (30) and the compressor element (1),and the space (25) is in connection with the inlet of the oil filter(19).
 12. Screw-type compressor according to claim 6, characterized inthat the control system (38) comprises a piston (37) which is movable ina chamber (43) and in one position forms a stop for atemperature-sensitive element (34) of the thermostatic valve (24). 13.Screw-type compressor according to claim 12, characterized in that thechamber (43), at one side of the piston (37), is in connection with theoil separator (10), such that the piston (37) can be maintained in saidposition by the pressure (P2) in this oil separator (10), and thecontrol system (38) comprises an auxiliary control in the form of arelief valve (47) putting the chamber (43) at said side in connectionwith the atmosphere when a control pressure (P1) is situated between twowell-defined values.
 14. Screw-type compressor according to claim 9,characterized in that the relief valve (47) is controlled by the controlpressure (P1) of the inlet valve (7).
 15. Screw-type compressoraccording to claim 13, characterized in that the relief valve (47)comprises a valve body (48) with a hollow part giving out to theatmosphere and which in its wall is provided with at least one opening(49) which, for a certain position of the valve body (48), gives outonto a duct (45) by which the chamber (43) is in connection with the oilseparator (10).
 16. Screw-type compressor according to claim 14,characterized in that at one extremity, the valve body (48) has apiston-forming part (48A) which is movable in a chamber (53) which is inconnection with a part of the compressor where the control pressure (P1)for opening the inlet valve (7) is prevailing.
 17. Screw-type compressoraccording to claim 16, characterized in that the other extremity of thevalve body (48) cooperates with two springs (56, 57), whereby the one(57) thereof is stronger than the other and only can be compressed bythe valve body (48) after the other (56) has been partially compressed.